The present disclosure relates to a filter circuit and a radio terminal including the filter circuit.
A mixer that converts the frequency of a high-frequency band signal and a bandpass filter that attenuates a disturbing wave other than a high-frequency band are generally mounted on a reception front-end unit of a radio communication apparatus. Since the bandpass filter includes a surface acoustic wave filter and a dielectric resonator, it is difficult to incorporate the bandpass filter into a high-frequency integrated circuit. In recent years, however, attempts to integrate a frequency conversion filter circuit combining two functions of the mixer and the bandpass filter into the front-end unit of a receiver have been reported, as shown in “Low-Power 2.4-GHz Transceiver With Passive RX Font-End and 400-mV Supply” by B. W. Cook, A. Berny, A. Molnar, S. Lanzisera, and K. S. J. Pister, Journal of Solid State Circuits, Vol. 41, No. 12, pp. 2757-2766, December 2006, “A 40 nm CMOS Highly Linear 0.4-to-6 GHz Receiver Resilient to 0 dBm Out-of-Band Blockers” by J. Borremans, G Mandal, V. Giannini, T. Sano, M. Ingels, B. Verbruggen, and J. Craninckx, International Solid-State Circuits Conference (ISSCC) Dig. Tech. Papers, pp. 62-63, February 2011, and “A Differential 4-Path Highly Linear Widely Tunable On-Chip Band-Pass Filter” by A. Ghaffari, E. Klumperink, B. Nauta, IEEE Radio Frequency Integrated Circuits Symposium (RFIC), pp. 299-302, May 2010.
According to “Low-Power 2.4-GHz Transceiver With Passive RX Font-End and 400-mV Supply” by B. W. Cook, A. Berny, A. Molnar, S. Lanzisera, and K. S. J. Pister, Journal of Solid State Circuits, Vol. 41, No. 12, pp. 2757-2766, December 2006, the frequency conversion filter circuit outputs an input signal multiplied by a local oscillation signal to a capacitance element connected to an output side and returns a frequency response of the capacitance element connected to the output side to the input side by shifting only the local oscillation frequency of the local oscillation signal to a highpass side. In the configuration and operation, the frequency conversion filter circuit operates as the same bandpass filter as a parallel LC filter resonating at a frequency of the local oscillation signal.